Referring to FIG. 1, a Delta robot 10 comprises a plurality of parallel drive arms 20 connected to an end effector 30 by means of rods 40. The rods 40 constitute together with the drive arms 20 and the end effector 30 parallelograms which restrict the movement of the end effector 30 to pure translation. Each drive arm 20 is driven by a servo motor through a gearbox, and the movement is transmitted to the rods 40 and ultimately to the end effector 30 via kinematic chains comprising ball joints 50 between the drive arms 20 and the rods 40, and the rods 40 and the end effector 30, respectively. A traditional Delta robot 10 has three drive arms 20 and three degrees of freedom, but variants of a Delta robot 10 may have four or more drive arms 20, and also more than three degrees of freedom. In the Delta robot 10 according to FIG. 1 a fourth degree of freedom is provided by a central shaft 60 transmitting rotating movement to the end effector 30.
When calibrating a drive arm 20 of a Delta robot 10, the respective drive arm 20 needs to be moved to a known position that is used as a reference position for the calibration. A conventional way of fixing a drive arm 20 into a reference position is to push the drive arm 20 all the way up until it comes into contact with a calibration pin 70 positioned at a robot base 80. While this reference position is accurate enough for many applications, for other applications it is not. The position of the calibration pin 70 in relation to the drive arm 20 depends on a chain of tolerances between the two components, and consequently the reference position is not as accurate as desired.
JP2009255197 discloses a jig that is configured to fix the drive arms of a Delta robot into a reference position. The jig is attached between a robot base and an end effector, and the reference position is transmitted to the drive arms via rods.
In practice a certain type of Delta robot has several configurations with different drive arm lengths and rod lengths (all the drive arms in a certain configuration having, of course, the same length, and all the rods in a certain configuration likewise having the same length). A drawback with the jig according to JP2009255197 is that when the drive arm length or the rod length changes, a use of the same jig results in a different reference angle value. There is therefore a desire to improve the existing methods of fixing a drive arm of a Delta robot into a reference position.